This invention relates generally to ion sources for mass spectrometers and more particularly to ion sources for mass spectrometers of the sector type. The invention is a result of a contract with the United States Department of Energy.
Sector mass spectrometers are used extensively for making analyses requiring highly precise determinations of the relative abundances of positive ions. The term "sector spectrometer" is used herein to refer to a mass spectrometer having parallel, generally sector-shaped magnet faces which cooperatively define a narrow elongated gap through which an ion beam is passed. Such spectrometers conventionally are designed with rectangular symmetry throughout, meaning that the various apertures or slits through which the ion beam is passed are of generally rectangular configuration.
FIG. 1 depicts a well-known modified Nier-type ion source which is designed with rectangular symmetry for use in sector mass spectrometers. As shown, the ion source is mounted adjacent to any suitable means 5 for generating positive ions. The source includes the following: a first electrostatic lens 7 for forming the positive ions into a beam and directing the same onto a first collimating slit 9; a second electrostatic lens (a pair of z-plates) 11 for positioning and focusing the beam in the z direction (shown in FIG. 1); and a second beam-collimating slit 13. More specifically, the beam-forming lens 7 comprises a case plate 15, a drawout plate 17, and a beam-alignment plate 19 for positioning and focusing.
Although ion sources of the Nier and modified-Nier types have been used extensively for many years, there has long been a need for even more efficient sources capable of providing lower limits of detection. In addition to being less efficient than desired, the conventional sources are relatively expensive to fabricate. Furthermore, they become contaminated with ion-beam products after relatively short periods of normal use.